1. Field
This invention relates to the field of bonding a collimator to an imager and, in particular, to using catch pads throughout the imaged area.
2. Related Art
Focal plane imaging sensors are well known for the purpose of acquiring an electronic image from a sensor array implemented on a semiconductor die. Exemplary imaging sensors include CMOS imaging arrays (hereinafter, CMOS imagers) and charge coupled devices (hereinafter, CCD).
Backthinning, removal of material from the backside of substrates, has been used to make the imaging sensors, particularly in the CCD applications. Performance advantages of backthinned sensors can include improved light sensitivity as a result of improved effective fill factor. When properly passivated by a method such as described in U.S. Pat. No. 5,688,715, the entirety of which is hereby incorporated by reference, backthinned CMOS sensors can demonstrate high sensitivity to UV light and low energy (˜0.5 to 20 keV) electrons and low energy X-Rays (<˜10 keV). The sensitivity to low energy electrons of backthinned CMOS sensors makes them particularly suitable for use in vacuum tubes as a video based image intensifier. U.S. Pat. No. 6,285,018 B1, the entirety of which is hereby incorporated by reference, details the use of a backthinned CMOS sensor in an electron-bombarded configuration. The backthinned CMOS sensor is mounted directly opposing a photocathode in a proximity-focused configuration.
Image intensifiers experience a modulation transfer function (MTF) degradation of sensor image associated with elastic scattering of electrons as the electrons strike the anode of the tube. In a proximity-focused tube, the scattered (including backscattered) electrons are attracted to, and re-impact the anode within a circle of radius equal to ˜2× the tube gap. This effect, often referred to as “halo,” is a particular problem when bright lights fall within the image intensifier field of view. There are a number of prior art approaches to minimize the impact of halo in image tubes incorporating a microchannel plate (MCP). U.S. Pat. No. 6,483,231 attempts to minimize halo in the cathode to MCP gap, and U.S. Pat. No. 5,495,141 attempts to minimize halo in the MCP to screen gap, the entireties of which are hereby incorporated by reference. However, in a tube without a microchannel plate, the image flux electrons lost in the collimator significantly reduce tube sensitivity. The glass draw technology described in U.S. Pat. No. 5,495,141 is typically limited to open area ratios on the order of ˜80%. This estimate is roughly consistent with the statement that gain lost in the collimator can be regained by increasing the applied voltage from 6000 to 10000 V.
Differences in the coefficient of expansion between the glass used to manufacture MCP-like structures and the silicon of CMOS die make it impossible to maintain pixel level alignments between a glass collimator and an electron bombarded active pixel imager over normal environmental temperature ranges. Modern dry etch technology is now capable of producing highly anisotropic etched structures in silicon. One method used to generate such structures is described in U.S. Pat. No. 5,501,893, the entirety of which is hereby incorporated by reference. U.S. Pat. No. 7,042,060 B2, the entirety of which is hereby incorporated by reference, describes collimator structures made using modern semiconductor techniques. Collimators made using anisotropic dry etching can exceed 90% open area ratio. Due to the exact coefficient of thermal expansion match between a silicon collimator and the silicon of a typical CMOS imager, silicon is typically used as the collimator material.
One consequence of the anisotropic-etch approach, and the high associated open areas, is that the collimator lacks the structural rigidity found in an MCP. U.S. Pat. No. 5,501,893 discloses placing the collimator in close proximity to the screen. The collimator is supported at the edge and spaced a few microns from the surface of the phosphor screen. U.S. Pat. No. 7,042,060 B2 describes multiple approaches including the use of a monolithic collimator, an edge supported collimator and a collimator bonded over the full active area. Modern image intensifiers employ various means to control sensor gain over widely varying input light levels. Direct view sensors use a combination of duty cycle gating of the cathode to MCP voltage and MCP voltage control to achieve suitable output light levels. Consequently, the electrostatic environment between the output of the MCP and screen, seen by the collimator described in U.S. Pat. No. 5,501,893, is constant during normal operation. When collimators are used in proximity focused solid state imaging sensors that do not employ an MCP, gain control is primarily achieved via duty cycle gating of the cathode-anode voltage. Consequently, the collimator is exposed to the alternating electrostatic field associated with the gating voltage. The collimator is conductive in order to maintain a drift field in the channels of the collimator. Similarly, the collimator is maintained at the anode potential during duty cycle gating. The electric field between the collimator surface and the photocathode results in a physical force that attracts the two surfaces toward each other.
In proximity focused solid state imaging sensors, optimum image resolution is achieved by minimizing photoelectron time of flight. Time of flight is in turn minimized by maintaining tight spacing between the cathode and anode. Spacing is typically limited by the required operational voltage of the sensor (typically between 500 and 8000V). The net result of close spacing and a relatively high, alternating electric field present at the surface of the collimator is that significant movement can occur in an edge supported collimator. Experience has shown that in the geometries used in commercial EBAPS sensors, this movement can exceed 100 microns. Increasing sensor gap by 100 microns is sufficient to measurably degrade sensor performance. Placing an edge supported collimator in close proximity to the anode in a gated electric field can result in physical damage to both the collimator and the anode. Similarly, momentum transfer from the moving collimator is transferred to the outside of the sensor vacuum envelope resulting in a measurable acoustic signature that changes with sensor varying high voltage gating conditions.
U.S. Pat. No. 7,042,060 B2 discloses two approaches that can address both the physical damage and acoustic signature issues. The first approach generates a monolithic collimator via the use of area selective backside thinning. In practice, it is a costly process to develop and a difficult process to implement at high yield. The cost can be minimized by fabricating discrete collimators via established semiconductor methods and subsequently bonding known good collimators to known good backside thinned imagers.
The second approach requires the collimator to be bonded to the underlying solid state imager throughout the active imaging area of the sensor via the use of a bonding medium. In the case of proximity focused solid state imagers that use GaAs, InGaAs, InP or other semiconductor photocathodes, ultra high vacuum (UHV) compatibility constraints apply to an acceptable bonding medium. The bonding media must both exclusively be composed of low vapor pressure materials and be compatible with the high temperature bake-out profiles necessary to achieve UHV pressures. The bond is made between the collimator and the passivated surface (also referred to as passivated layer) of the back-side thinned semiconductor imager, and this passivation layer is required to achieve good collection of electrons generated near the back surface of the imager. In order to achieve the best possible performance, the passivation layer is quite thin (typically <˜500 Angstroms). Consequently, any interaction or contamination of the surface of the backside thinned imager resulting from the presence of the bonding media during UHV thermal processing may result in performance degradation of the sensor. Compatibility of the bonding media with the passivation layer represents an additional constraint on the bonding media.
In view of the above, a method and system are required to bond the collimator to the solid state imager that does not suffer from these drawbacks.